Method and apparatus for detecting hydrocarbon fuels in a vapor state with an absorber-expander member

ABSTRACT

A method and apparatus for detecting the presence of hydrocarbon analyte in at least one of a liquid and vapor state. The apparatus (21) includes an optical fiber (22), an absorber-expander (31) mechanically coupled (32) to the optical fiber (22) to produce a change in transmission of light along the fiber (22) upon absorption of the hydrocarbon (33). The absorber-expander (31) is selected to absorb hydrocarbons, but not water, and to be capable of multiple reversible expansion and contraction cycles without significant structural degradation. Methyl terminated, silica and iron oxide filled, dimethyl polysiloxane provides a material which will experience substantial swelling in the presence of a hydrocarbon and yet is substantially reversible on desorption of the hydrocarbon from the absorber-expander (31). The method includes positioning an optical fiber (22) having such an absorber-expander (31) for absorption of hydrocarbons and detecting the decrease in light transmission produced by expansion of the absorber-expander (31), for example, by one of microbending and axial misalignment. The location of the absorber-expander (31) along strand (22) can be determined by optical time domain reflectometry or by digital sensing nodes (122), and multiple couplings (57) and biasing couplings (153) for mounting the absorber-expanders (152) to the optical fiber (151) can be used to enhance sensitivity.

TECHNICAL FIELD

The present invention relates, in general, to the detection ofhydrocarbon analytes in either a liquid or vapor state, and moreparticularly, relates to a method and apparatus for the detection ofhydrocarbon fuels employing microbending of an optical fiber ormisalignment of fiber portions and optical time domain reflectometry.

BACKGROUND ART

The detection or sensing of hydrocarbon fuels, such as diesel oil,gasoline and Jet-A fuel leaking from storage tanks has receivedconsiderable attention. These hydrocarbon fuels are stored insubstantial volume in above ground and below ground storage tanks andpresent a significant hazard to safety and health if they leak into thesurrounding environment.

Two approaches have generally been taken to the problem of hydrocarbonfuel leakage. First, storage tanks may be constructed with doublebottoms so that leakage from the inner tank is caught and contained bythe outer bottom wall. This approach is very expensive and is oftenimpractical in retrofitting situations. The second approach is toprovide detection or sensing apparatus proximate the storage tanks whichare capable of sensing leaking fuel from the tanks. Upon detection ofleaking fuel, the source of the leak can be found and repaired. Theseapproaches also may be used together.

The detection of leaking hydrocarbon fuels, however, is not withoutconsiderable problems. Tanks themselves often are very large andsituated in even larger tank farms, making it necessary for amultiplicity of detectors to be used and a premium to be placed onlocating the source of the leak. As a plurality of discrete detectionapparatus are employed, the detection costs rise rapidly. If multipledetectors are not used, sufficient oil may leak to the surroundingenvironment so as to present a substantial health and/or safety hazardbefore detection occurs. Moreover, as more fuel escapes the location ofthe leak will be more difficult to determine.

Hydrocarbon leak detecting apparatus often have been constructed in amanner which requires their replacement or repair upon detection of aleak, that is, once contacted by a hydrocarbon fuel, the detectiondevice, or its key components, must be replaced before the detector canbe used again. Another problem is that in most storage tank farms, therewill be considerable ground water present, and any detector must becapable of distinguishing between ground water and hydrocarbons andcapable of functioning without being overwhelmed by ground water inorder to avoid false detection signals. Finally, most hydrocarbondetectors are based upon sensing hydrocarbons in either a vapor state ora liquid state, but not both. The vapor-based sensors, therefore, tendto be overrun by ground water and liquids, and the liquid-base sensorstend to be insensitive to the presence of vapor.

In recent years, many attempts have been made to employ the unique andvaried light transmission properties of optical fibers in detectingapparatus. In communication cable applications, the microbending of anoptical fiber has been used to detect the location of moisture or groundwater entering the cable. In U.S. Pat. No. 4,596,443 to Diemeer, et al.,an unspecified swelling agent is positioned inside the cable and ismechanically coupled to press a ribbed-shaped pattern against theoptical fiber upon swelling of the agent. Optical time domainreflectometry or a back-scatter technique is used to locate the positionof the microbend along the fiber, and thus the position of water leakinginto the cable. Such a system, however, is designed for sensing thepresence of water, rather than to be insensitive to water and to detectthe presence of hydrocarbon fuels. Reversibility of the swelling agent'sexpansion also is not suggested or disclosed in the Diemeer, et al.patent.

U.S. Pat. No. 4,590,462 to Moorehead also employs microbending of anoptical fiber in a detection unit, and the Moorehead device is used todetect hydrocarbon fuels. A rotary actuator is mechanically coupled toan optical fiber to produce microbending of the fiber. The rotaryactuator includes a spring mechanism having stored energy which isreleased upon degradation of shear pins under the action ofhydrocarbons. Thus, when the hydrocarbon analyte is present insufficient quantity to degrade the shear pins, the spring is releasedand the optical fiber displaced to produce a microbend that can besensed by optical time domain reflectometry. This approach, however,clearly is not reversible since it depends upon destruction of the shearpins upon contact with the hydrocarbon.

A number of prior art fiber optic-based detector systems have been basedupon the coupling of the evanescent wave traveling down the exterior ofthe fiber optic core. Thus, U.S. Pat. Nos. 4,270,049, 5,138,153,5,144,690 and 5,168,156 are all based on evanescent wave phenomenon. Inthe patent to Tanaka et al., U.S. Pat. No. 4,270,049, a fiber opticsensor assembly is employed in which the light transmitted down thefiber optic core is reduced in intensity due to adhesion of an analyte,such as a hydrocarbon fuel, to the core. The core is clad with materialhaving an index of refraction which is less than the core index ofrefraction, and contact and adhesion of the analyte to the claddingresults in an increase in clad index of refraction which results in areduction in the light transmitted along the core. The Tanaka et al.patent also teaches the use of silicon resins as a cladding which willbe damaged or broken down by hydrocarbons.

In U.S. Pat. No. 5,138,153 to Gergely et al., a distributed fiber opticsensor based upon evanescent effects is disclosed in which the caddinghas an index of refraction less than the core and the cladding issensitized to the analyte. When the analyte contacts the cladding, itincreases the index of refraction of the cladding above the core tothereby couple the light transmitted in the core to the evanescent wave.The Gergely et al. patent employs its sensor system in a hydrocarbontank farm, but the cladding is selected to undergo an increase in theindex of refraction. Optical time domain reflectometry is used to locateleaks, and both continuous and pulsed light can be employed to senseliquids and vapors having analytes which will react with the cladding.The patent to Gergely et al., however, has no disclosure as to claddingmaterials which are suitable for use on the fiber optic strand.

U.S. Pat. No. 5,144,690 to Domash discloses a fiber optic sensor systemin which coating patterns are provided which induces strain on a dualcore optical fiber. Evanescent wave coupling between cores is sensed.This system is not disclosed as being intended for hydrocarbon fueldetection.

U.S. Pat. No. 5,168,156 to Fischer et al. employs a fiber optic sensorassembly in which three fibers are used with one acting as an input, andthe other two acting as a reference fiber and a signal fiber. The threefibers are coupled together optically and the sensor fiber is strippedwith cladding and exposed to an analyte to be sensed. Light attenuationas a result of the analyte affect on the evanescent wave on the uncladfiber is detected as compared to the reference fiber, which is clad andshielded from the analyte.

Fiber optic detectors also have been based upon the interpositioning ofa sensor material along the length of a fiber optic core so thattransmission and/or reflection measurements will indicate when ananalyte is present at the material interposed along the core. U.S. Pat.Nos. 4,842,783, 5,015,843 and 5,164,588 are examples of this approach.In U.S. Pat. No. 4,842,783 to Blaylock, a fiber optic sensor assembly isprovided in which a polymeric gel is cross-linked in situ onto the endof the fiber and a dye absorbed into the gel, which preferably isswellable to absorb the dye. The dye in the gel is selected to beresponsive to an analyte to be sensed. The dye, for example, can befluorescent but the system is not designed for use with hydrocarbons.Dyes, however, tend to be suitable only in irreversible chemicalreaction which require replacement of the sensor once the analyte isexposed to it.

U.S. Pat. No. 5,015,843 to Seitz et al. is directed to a fiber opticsystem in which polymer swelling is used to mechanically or physicallydisplace a reflective surface coupled to the fiber optic core andthereby influence light transmission back to the detector. The systemrequires a relatively high concentration of analyte to be effective, andin order to enhance a sensitivity and minimize this disadvantage, thesystem preferably is miniaturized. Numerous polymers are discussed forsensing various products, none is disclosed as being reversible in itsreaction with hydrocarbon fuels.

In U.S. Pat. No. 5,164,588 to Marcus, a distributed sensor system isprovided in which reflector/transmission couplings and analyte sensorsare interposed in series and alternating along an optical fiber strand.Light pulses pass through the sensors, and in part through theconnectors and in part back to a detector, and the pulses can be used todetect environmental or analyte effects on the sensors. Many analytesmay be sensed but hydrocarbon fuel sensing is not disclosed.

Notwithstanding the success of the various optical fiberbased detectorsin detecting a wide range of analytes, detecting the presence of liquidand/or vapor hydrocarbon fuels, and distinguishing the same from groundwater, by a detector system which is reversible and reusable throughmany cycles without significant hysteresis loss remains a substantialproblem.

An alternative to optical fiber detection has also been employed whichis currently being marketed under the trademark GORE-TEX cable, forexample, by W. L. Gore and Associates of Phoenix, Ariz. The GORE-TEXcoaxial cable has a hydrocarbon fuel absorbent media (expanded PTFE)situated between a central conductor and an outer, perforated,cylindrical conductor or shield. Pulses of RF energy are transmitteddown the coaxial cable and are reflected back at the location ofabsorbed hydrocarbon. Time domain reflectometry is used to locate theposition along the cable at which hydrocarbons are absorbed.

The GORE-TEX cable, however, cannot distinguish between Jet-A, gasolineor diesel fuel. It will not detect hydrocarbons in a vapor state, itcannot generate an analog output signal, and it has location resolutionand distance range limitations.

Accordingly, it is an object of the present invention to provide anapparatus and method for detecting the presence of hydrocarbon fuels ineither a liquid or a vapor state which can discriminate between suchfuels and water and is suitable for use for multiple detection cycles.

A further object of the present invention is to provide a hydrocarbonfuel detection apparatus and method which is easy to install, requiresminimum maintenance, is inexpensive to construct, is easily adjusted andcan differentiate between different hydrocarbon fuels and candistinguish such fuels from ground water.

The hydrocarbon fuel detection apparatus and method of the presentinvention has other objects and features of advantage which will becomeapparent from, and are set forth in more detail in, the accompanyingdrawing and the following description of the Best Mode of Carrying Outthe Invention.

DISCLOSURE OF INVENTION

A method for detecting the presence of a hydrocarbon in at least one ofa liquid and vapor state is provided which is comprised, briefly, of thesteps of selecting an absorber-expander material which is hydrophobicand yet absorbs hydrocarbon fuels and expands by a significant amount.The absorber-expander member must be capable of multiple expansion andcontraction cycles in the presence and absence of the hydrocarbonanalyte, in either a liquid or vapor state, while remainingsubstantially undegraded, and red silicone rubber meets these criteria.The next step is to mechanically couple the silicone rubber member to afiber optic strand in a manner producing a decrease in light intensitytransmitted along the strand, for example by microbending of the strandupon expansion of the silicone rubber member or axial misalignment ofstrand portions. Finally, the method includes the step of opticallycoupling detection apparatus to the strand for detection of theoccurrence of a reduction in light transmission along the strand.

The detection apparatus of the present invention is comprised, briefly,of an optical fiber, and an absorber-expander mechanically coupled tothe optical fiber to produce a change in transmission of light along thefiber upon absorption of a hydrocarbon, with the absorber-expander beingformed from a hydrophobic hydrocarbon-absorbing rubber material selectedto expand upon absorption of the hydrocarbon and retain sufficientstructural integrity to permit repetitive expansion and contractioncycles, and detection apparatus optically coupled to the fiber fordetection of a change in light transmission in the fiber due tomicrobending.

DESCRIPTION OF THE DRAWING

FIG. 1 is a top plan, schematic representation of a hydrocarbondetection apparatus constructed in accordance with the presentinvention.

FIG. 2 is an enlarged end elevation view, in cross section, takensubstantially along the plane of line 2--2 in FIG. 1.

FIG. 2A is a cross-sectional view corresponding to FIG. 2 of analternative embodiment of the apparatus of FIG. 1.

FIG. 2B is a cross-sectional view corresponding to FIG. 2 of still afurther alternative embodiment of the apparatus of FIG. 1.

FIG. 3 is a top plan, schematic representation of an alternativeembodiment of the detector apparatus of the present invention.

FIG. 4 is a top plan, schematic representation of still a furtheralternative embodiment of the detector apparatus of the presentinvention.

FIG. 5 is a side elevation view, partially in cross-section, of anotheralternative embodiment of the detector apparatus of the presentinvention.

FIG. 5A is a schematic side elevation view of a further alternativeembodiment of the detector apparatus of the present invention.

FIG. 5B is an end elevation view of the apparatus of FIG. 5A takensubstantially along the plane of line 5B--5B in FIG. 5A.

FIG. 6 is a top plan, schematic representation of still anotheralternative embodiment of the apparatus of the present invention.

FIG. 7 is a top plan, schematic representation of an assembly ofdetectors constructed in accordance with the present invention andarranged to provide digital location signals

FIG. 8 is a fragmentary side elevation view of an alternative embodimentof the apparatus of the present invention suitable for adjustablebiasing of the detector sensitivity.

FIG. 9 is an end elevation view of the apparatus of FIG. 8.

FIG. 10 is a graphical representation of transmissivity versusdisplacement for the apparatus of FIG. 8.

BEST MODE OF CARRYING OUT THE INVENTION

The hydrocarbon analyte detection apparatus of the present invention isconstructed in a manner which allows it to be capable of distinguishinghydrocarbons from water, in both liquid and vapor states, and to becapable of multiple detection and restoration cycles. Thus, the presentdetector and method do not require extensive repair or replacement ofcomponents upon detection of the target hydrocarbon analyte, such ashydrocarbon fuels. This allows the detection apparatus of the presentinvention to be positioned in difficult to access locations with littleor no repair or replacement being required. Once the leaking hydrocarbonanalyte has been contained and removed through absorption, evaporationand/or other techniques, the present detection apparatus willautomatically evaporate absorbed hydrocarbons and be restored to acondition capable of detecting the next instance of spilling or leakingat the detector location.

Referring now to FIG. 1, a detection apparatus, generally designated 21,is shown. The present detection apparatus includes an optical fiber orstrand 22 which is optically coupled to a source of light energy 23 andis further optically coupled to a detection apparatus or detector 24.While illustrated in FIG. 1 in an arrangement in which source 23 iscoupled to end 26 of fiber 22 and detector 24 is coupled to opposite end27 of fiber 22, it will be understood that using conventional fiberoptic techniques the detector and source can be coupled to the same endof the fiber.

The composition of fiber 22, the components in source 23 and detector 24are not regarded as a novel portion of the present invention, and theyare well-known in the fiber optic industry. Source 23, for example, canbe a LED source, a laser, or incandescent light bulb. Detectionapparatus 24 is shown in FIG. 1 as a light sensing apparatus capable ofsensing a decrease in light flux transmitted from source 23. Suchdetection apparatus, for example, is commercially manufactured by EG andG Vactec of St. Louis, Mo. and sold under Model No. VTP 8440. Opticalfiber strand 22 can be any typical communications optical fiber.

In order to produce a detectable decrease in light intensity sensed bydetector 24, detection apparatus 21 of FIG. 1 includes anabsorber-expander member 31 mechanically coupled, in this case by ringmember 32, to fiber 22 in a manner which will produce microbending offiber 22 upon absorption of a hydrocarbon analyte. In the embodimentshown in FIG. 5, an absorber-expander is coupled to one of two portionsof an optical fiber in order to decrease the transmitted light intensityby misaligning the axes of the fiber portions.

The selection of absorber-expander 31 is a very important feature of thepresent invention, whether microbending or misalignment are produced byexpansion of the absorber-expander. The absorber-expander must be ableto distinguish between ground water and hydrocarbons in both liquid andvapor states. It must be reversible, that is, capable of multipleexpansion and contraction cycles upon absorption of hydrocarbon and thensubsequent desorption of the hydrocarbon from the absorber-expander.

The problem of selecting an absorber-expander material 31 which iscapable of multiple cycles of expansion and contraction is substantialsince many compounds are attacked by hydrocarbon analytes andparticularly hydrocarbon fuels. Thus, these fuels act as a solventand/or cause serious degradation of numerous potential absorber-expandermaterials.

It has been found, however, that dimethyl polysiloxane rubber, which ismethyl terminated and has silica and iron oxide fillers, is not onlyhydrophobic, which is required to prevent ground water absorption, butit is capable of absorbing many hydrocarbon fuels. Moreover and veryimportantly, such silicone rubber expands upon absorption ofhydrocarbons and contracts as the fuel leaves or is desorbed leaving theabsorber-expander in substantially its original condition. Methylterminated, and silica and iron oxide filled, dimethyl polysiloxane iscommercially distributed under the name of Red Silicone Rubber and isproduced commercially by companies such as General Electric Companythrough its GE Silicone division.

Red Silicone Rubber can be cast or extruded in virtually any shape toproduce a self-supporting member that is easily coupled to fiber opticstrand 22. Silicone rubber will not absorb water, and accordingly, it iscapable of detecting hydrocarbon fuels even when buried in ground whichperiodically or perennially has substantial ground water. Thisabsorber-expander material can even be used in a water environment. WhenRed Silicone Rubber is exposed to the hydrocarbon fuels such asgasoline, diesel oil, Jet-A fuel, either in a liquid or a vapor state,such hydrocarbons will be absorbed and produce substantial expansion ofthe member 31. Thus, as much as 40% expansion will occur when RedSilicone Rubber is allowed to absorb gasoline, 35% when absorbing dieseland 20% when absorbing Jet-A fuel.

As shown in FIGS. 1 and 2, therefore, a body of hydrocarbon analyte orfuel 33 can be seen to contact or be in sufficiently close proximity toabsorber-expander 31 that liquid and/or vapor will be absorbed by member31. Such absorption produces substantial expansion of member 31, whichin turn is confined by containment or coupling ring 32. The result isthat the fiber 22 is bent upwardly, as viewed in FIG. 1, and to theleft, as viewed in FIG. 2, by the expansion of member 31 throughabsorption of hydrocarbon 33. The microbending at coupling ring 32 willresult in a reduction in the amount of light transmitted from source 23to detector 24. Thus, the sensing of a decrease in light transmission bydetector 24 can be used to detect the presence of a hydrocarbon analyteat absorber-expander 31. In order to be able to detect fuel leakagearound large storage tanks or in tank farms, a plurality of detectorsconstructed as shown in FIG. 1 could be used, but it is preferable todistribute a plurality of members 31 along the length of a commonoptical fiber 22. Thus, members 31 can be secured by coupling rings 32along the length of a fiber optic strand 22 which is positioned adjacentand/or under hydrocarbon fuel storage tank. The presence of leaking fuelat any of the discrete detectors will cause expansion of the RedSilicone Rubber members 31 and microbending of the fiber optic strand.Detector 24 can sense the occurrence of microbending somewhere along thestrand and an optical time domain reflectometry detector, schematicallyshown in FIG. 4, can be used to determine which absorber-expander issensing a leak.

As shown in FIG. 2, the fuel leak 33 is supported on a surface 34 whichis impervious to the fuel. This condition can occur, for example, whenthe tank is surrounded by a fuel-impervious surface, but in manyinstances, surface 34 will be porous to fuel 33, and the fuel will notmigrate from the storage tank to a position above the ground until thequantity of fuel leaking is substantial. Thus, the ability to absorbvapor coming up through a porous surface 34 allows the detector of thepresent invention to be easily retrofit to sense oil or hydrocarbon fuelleakage around storage tanks by simply placing the detectors aboveground immediately proximate the tank. In an original installation,however, it also can be advantageous to bury the strand portion ofdetector assembly 21 and its mechanically coupled absorber-expanders 31so that direct contact and more rapid absorption occurs.

As the leaking fuel is removed by absorption, evaporation and similaratmospheric and/or remediation steps, the hydrocarbon absorbed by RedSilicone Rubber absorber-expander 31 will also desorb from or diffuse orevaporate out of the absorber-expander. This desorption gradually causescontraction of the absorber-expander and the microbending of fiber 22diminishes. Depending upon the location of absorbent-expander 31,virtually complete desorption of hydrocarbon analyte from the RedSilicone Rubber will occur. In below-ground installations, this processtakes longer, but detector apparatus 24 can sense not only the presenceof hydrocarbon, but also the degree of microbending, and accordingly theconcentration of the hydrocarbon analyte. Thus, when the process startsto reverse itself, detector 24, as well as the OTDR, can detect or "see"the reduction in microbending as analyte is desorbed fromabsorber-expander 31. Accordingly, after remediation has been completed,the new level of light transmitted along fiber 22 can be used as a newdetection threshold, which can be periodically lowered to reflectcontinuing desorption of the hydrocarbon from the absorber, and any newleak will cause the absorber-expander to begin to expand again,producing microbending and dropping of the light intensity below thethreshold. In above-ground installations, Red Silicone Rubberexpander-absorbers 31 will return to their original condition with verylittle hysteresis.

In FIG. 2A, an alternative embodiment is shown in which fiber opticstrand 22a is mechanically coupled to Red Silicone Rubberabsorber-expander 31a by a containment staple 32a. Absorber-expander 31ais shown with a rectangular cross-section which presents more surfacearea for contact with hydrocarbon 33a. Again, expansion of theabsorber-expander pushes fiber 22a to the left in FIG. 2A aroundopposite sides of staple 32a to create a detectable microbend.

In FIG. 2B, still a further alternative embodiment is illustrated inwhich rectangular absorber-expander 31b is coupled to fiber optic strand22b by a trapezoidal-shaped containment or coupling ring 32b. In orderto protect strand 22b against damage during handling, a flexiblecaulking compound 36 has been applied over the strand 22b. Caulking 36does not function as an expander, nor is it used to mechanically containor restrain the fiber. It merely functions to provide abrasion andinstallation handling protection and to adhere the fiber to the expanderas a unit. It is not even essential that the caulking be resistant tohydrocarbon fuels, since once in place the optical fiber is notroutinely exposed to being damaged. The preferred caulking material 36is an RTV adhesive silicone sealant Dow-Corning 748.

In order to illustrate the substantially hysteresis-free cyclingpotential of methyl-terminated, silica and iron oxide filled siliconerubber, the following is an example using an absorber-expanderconstructed as shown in FIG. 1.

EXAMPLE 1

A Red Silicone rubber absorber-expander and having a diameter of 3/8inches, a length of 1.0 inches was attached to a 100/140 optical fiber.A 4100 mCd LED was used as a source and photo diode used as a detector.The absorber-expander was placed in a pool of standing gasoline with theabsorber immersed by about 0.001 inches 10 minutes. The light signalmeasured by the sensor was initially 2 volts and after 10 minutes haddropped to 1.1 volts. The absorber-expander was then removed from thegasoline and allowed to desorb for 150 minutes. The measured lightsignal increased to 2 volts. The same absorber-expander was cycled 8times for the same length absorption desorption cycles, and on the lastcycle the measured light signals were within 99% of the first cycle.

EXAMPLE 2

The same assembly as described in Example 1 was placed in a pool ofwater and allowed to stand for 1000 minutes. Light intensity readingswere taken every 100 minutes as follows:

1. 2 volts

2. 2 volts

3. 2 volts

4. 2 volts

The detector was then placed in a pool of diesel oil and after 30minutes the sensed flux had dropped to 28%. The detector was thenremoved from the oil and allowed to stand in air for 400 minutes untilthe light measured by the detector increased to 95. At this point, thedetector was placed again in the pool of water for 1000 minutes and thelight transmission continued to improve up to approximately 99%.

EXAMPLE 3

The assembly of Example 1 was positioned inside a metal container andburied in uncontaminated soil. The container was 12 inches by 24 inchesand 4 inches deep. At one end of the container, 3 cubic centimeters ofgasoline was poured slowly into the container and measurement of thelight signal at the detector yielded the following:

    ______________________________________                                        1.         0 minutes      2 volts                                             2.        30 minutes      1.9 volts                                           3.        60 minutes      1.6 volts                                           4.        90 minutes      1.4 volts                                           ______________________________________                                    

Referring now to FIG. 3, an alternative embodiment of the apparatus andmethod for detecting hydrocarbon analytes is shown. In FIG. 3, lightsource 41 is coupled to a plurality of optical fibers, in this casethree fibers 42, 43 and 44. Mechanically coupled to fibers 42, 43 and 44are absorber-expanders 46, 47 and 48 which are each capable of multiplereversible expansion and contraction cycles upon absorption anddesorption of hydrocarbon analytes. Detectors 51, 52 and 53 areoptically coupled to fibers 42, 43 and 44, in this case at end oppositesource 41. The detectors in turn are electrically connected byconductors 54 to a calculating chip or computer 56 for receipt ofsignals from the detectors.

As will be seen, each of the absorber-expanders 46-48 is connected totheir respective fiber optic fibers by a plurality of coupling rings 57.The use of a plurality of rings as a coupling mechanism around theoptical fiber has the effect of increasing the sensitivity of eachsensor element. The additional rings cause the system to function on thesame basis as a so-called multiple-pass spectrometer. That is, the fluxin the optical fiber is utilized repeatedly, in this case three times,as it passes through each "microbent" section of the optical fiber. Theimprovement in signals is approximately equal to the product of theeffect of each ring 57 on the optical fiber.

In the sensor assembly of FIG. 3, it is preferable thatabsorber-expanders 46-48 be formed of rubber polymers which will absorbdifferent hydrocarbon fuels and expand by amounts which are sufficientlydifferent to enable assembly of expanders 58 to distinguish betweendifferent hydrocarbon fuels. Thus, expander 46 may advantageously beprovided as Red Silicone Rubber, while expander 47 is Norprene rubberand expander 48 can be Latex rubber.

A mathematical formula has been derived which uses as input signals, thetime rates of expansion of the absorbers to determine which hydrocarbonis present. For example, software such as MATHEMATICA algebraic softwarecan be used to generate an algorithm which will fit into a calculatorchip.

Detectors 51-54, therefore, can provided outputs to computer orcalculator chip 56 in which the expansion of all threeexpander-absorbers in the presence of an unknown hydrocarbon fuel can becompared, and the relative expansion rates used to distinguish thehydrocarbon fuel present at assembly 58.

The following Table 1 shows the expansion rates for each absorbermaterial for the three most common hydrocarbon fuels.

                  TABLE 1                                                         ______________________________________                                                     Gasoline Diesel  Jet-A                                           ______________________________________                                        Red Silicone Rubber                                                                          27         6       11                                          Norprene Rubber                                                                              16         3       7                                           Latex Rubber   20         4       2                                           ______________________________________                                    

In FIG. 4, the detector apparatus of the present invention includes aplurality of absorbers-expanders 61, 62 and 63 mechanically coupled by awire wrap element 64 to optical fiber 66 to form a sensor node assembly,generally designated 67. Each of the absorber-expanders is coupled by aplurality of wraps of wire members 64 so as to enhance sensitivity in amanner described above, and further the presence of three sensorabsorbers in series on the same optical 66 increases the sensitivity ofnode 67.

In the assembly of FIG. 4, source 68 transmits light through fiberbranch 66a of optical fiber 66 to a splitter 69 and a detector, notshown, is optically coupled to the opposite end 71 of fiber 66 andformed to sense the decrease in intensity produced by microbending atnode 67. It is contemplated that there will be a plurality of nodes 67located in series along fiber 66. Accordingly, determination of whichnode 67 along the optical fiber is producing microbending can beaccomplished by providing an optical time domain reflectometry (OTDR)back-scatter measuring device 72, which is coupled by fiber branch 66bto optical splitter 69, in a manner well-known in the art. A computingdevice or computer 73 can be coupled to OTDR detector 72 by conductor 74for receipt of signals therefrom. Thus, the OTDR detector senses thebackward propagation of Raleigh scattered optical flux as a result ofmicrobending of the fiber. This allows a determination to be made bycomputer 73 of the location along the microstrand of the sensing node atwhich microbending is occurring. The detector end 71 of the strand isused as an inexpensive sentinel, only for the purpose of alertingpersonnel of a problem, whereupon an OTDR (very expensive) is broughtinto use for locating and monitoring the analog behavior of each sensor.

The use of OTDR to locate the sensor causing a light transmissiondecrease can be applied to virtually all of the illustrated detectorswhen multiple detector nodes are present in series on the same fiber orfibers.

In the embodiments of the present invention shown in FIGS. 5, 5A and 5Bother manners of mechanically coupling an absorber-expander to anoptical fiber in a way which will produce a decrease in lighttransmission on absorption of a hydrocarbon is shown. The optical fiberin FIG. 5 is comprised of a first fiber portion 81 and a second fiberportion 82, which are mounted in a framework 83 so that the opposed ends86 and 87 are axially aligned. Source 88 will therefore transmit downfirst fiber portion 81 light flux which is transmitted across gap 89 tosecond fiber portion 82 and thereafter to detector 91. Anabsorber-expander 92 is positioned between framework 93 and the opticalfiber, in this case the second fiber portion 82. Framework 83 ispreferably an open framework which provides easy access of liquid andvapor hydrocarbon fuels to absorber-expander 92.

Upon absorption of the hydrocarbon into absorber-expander 92, theexpansion will cause transverse or lateral displacement of secondoptical fiber portion 82, which results in an axial misalignment of ends86 and 87 so that the light transmitted along fiber 81-82 diminishesintensity.

Aperture 89, however, can be influenced by humidity in the form ofcondensation in the aperture during temperature decreases. In order toeliminate such condensation problems, the assembly of FIG. 5 includes atubular shielding element 93 which loosely receives the ends 86 and 87of fiber portions 81 and 82. Disposed in tube 93 can be a stableencapsulating liquid 94, such as silicone oil. It is preferable thattubular shielding member 93 be of sufficient size so that silicone oil94 will be trapped between the fiber and tube by surface tension forces.Thus, no enclosure is required between tube 93 and end 87 of secondfiber portion 82, which is free to be displaced transversely relative toend 86 of first fiber portion 81. This assembly, with the shielding tubeand stable inert oil, will keep condensation from occurring at gap 89and cause the transmission across the gap to be the same except foraxial misalignment of fiber portions of 81 and 82 caused by expansion ofmember 92.

Another feature of the assembly of FIG. 5 is that expander 92 can bemoved by the user from its solid line position to the position shown indotted lines in FIG. 5. The position along frame 83 of expander 92 canbe adjusted to adjust the sensitivity of the detector assembly. In thesolid line position of FIG. 5, absorber-expander 92 will produce greaterlateral deflection for each unit of expansion than will occur in thedotted line position in FIG. 5 due to its position close to fibersupport point 96 on frame 83. Thus, the combination of varying the pointat which expansion is applied relative to the support point 96 and therather rapid light transmission drop produced by misalignment, enhancethe signal-to-noise ratio of the assembly over the microbendingdetectors described above. Again, however, Red Silicone Rubber has theadvantage of being reversible substantially without degradation andhysteresis loss in the assembly of FIG. 5.

The optical fiber detector assembly of FIGS. 5A and 5B also includes afirsts fiber portion 81a and a second fiber portion 82a, which hasaxially aligned opposed ends 86a and 87a. Source 88a will thereforetransmit down first fiber portion 81a light flux which is transmittedacross gap 89a to second fiber portion 82a and thereafter to detector91a. An absorber-expander 92a is positioned between opposed, fixedsupport surfaces 93a and 93b and has optical fiber 81a, 82a cast in it.

As will be seen from FIGS. 5A and 5B, the preferred form ofabsorber-expander 92a is to include two off-set masses which arerelatively thin in cross section so that upon absorption of thehydrocarbon into absorber-expander 92a, the expansion will causetransverse or lateral displacement of optical fiber portions 81a, 82a inopposite directions. This is shown in dotted lines in FIG. 5A andresults in an axial misalignment of ends 86a and 87a so that the lighttransmitted along fiber 81a, 82a diminishes in intensity.

In the assembly of FIGS. 5A and 5B, fiber 81a, 82a can be cast into theabsorber-expander body and then slit at 89a from one side of body 92a.More accurately, fiber 81a, 82a is notched while in the body 92a andthen cleaved at the notch to form slit 89a. This approach easesmanufacture and produces an assembly in which gap 89a is not easilyinfluenced by humidity in the form of condensation in the apertureduring temperature decreases. The slit in body 92a produced by thenotching tool closes upon removal from the absorber-expander.

A temperature compensated form of the detector of the present inventionis shown in FIG. 6. Light source 101 transmits a signal down opticfibers 102 and 103 and absorber-expander 104 is mechanically coupled tofiber 102 by ring 107. A temperature compensating member 106 ismechanically coupled by ring 108 to fiber 103. Detectors 109 and 111sense light transmission intensity and are input through conductors 112to a computer or calculator chip 113.

In the assembly of FIG. 6, absorber-expander 104 in the sensing node,generally designated 114, can be Red Silicone Rubber while the othermember 106 is a material selected as a reference material that willexpand as a function of temperature in a manner which is known inrelationship to the expansion of Red Silicone Rubber. The second member106, however, is also selected so that it will not absorb to anysignificant degree or expand in the presence of hydrocarbon fuels.

One material suitable for use as the second or temperature compensatingmember 106 is Buna-N rubber. The temperature co-efficients of expansionof Red Silicone Rubber and Buna-N rubber are very similar (namely,0.00032 and 0.00035, respectively) and by properly dimensioning physicalsizes of members 104 and 106 the temperature expansion effects can bemade to be substantially identical. Since hydrocarbons will effect onlysensor element 104, the result will be a finite "difference" signal onlywhen the hydrocarbon fuel is present at sensor node 114. Thus, two fiberoptic signal channels can be used and compared to determine moreaccurately the value of the vapor or liquid density which has causedabsorber expansion by being able to subtract out, or ratio out, thetemperature expansion effects.

As will be appreciated, nodes identical to node assembly 114 can bepositioned along the two-channel fiber optic assembly of FIG. 6 in amanner similar to that above described. When multiple sensor nodes arepresent, an optical time domain reflectometry detector would be employedto determine the location as between sensing nodes. Computer 113 wouldbe used to obtain a temperature-insensitive value for the vapor orliquid sensed at the sensor node. Other assemblies of absorber-expandersand temperature expanders can be provided so that the mechanicalcomponents have motions which balance out to zero net displacement,while hydrocarbon absorption produces unbalanced expansion. Suchtechniques are particularly easily adapted to fiber misalignmentdetector assemblies and are analogous to Wheatstone bridge hullingcircuits.

A combination of optical fibers and absorber-expanders also can be usedwithout optical time domain reflectometry to provide locationinformation by using sensor assembly nodes capable of providing digitaloutput information. In FIG. 7, a circular oil tank 121 has a pluralityof sensor assemblies or nodes 122 positioned around the periphery of thetank. In this case, there are 13 sensor nodes with 12 being positionedproximate the periphery and 1 proximate the center of the tank, but itwill be appreciated that many other configurations are possible. Eachsensor node 122 is connected to four optical fibers 123-126.Mechanically coupled to at least one of fibers 123-126 at each sensornode 122 is an absorber-expander 131, for example, by a ring 132 orsimilar coupling as above described. The number of absorber-expanderscoupled to the optical fibers, and the optical fibers which theabsorber-expanders are coupled to are selected to provide a distinctivedigital output for each node 122 of the series sequence of sensor nodes.

Optically coupled to fibers 123-126 is a source of light flux 133 andlight intensity detectors 136-139. A computer assembly or calculatingchip 141 can be electrically connected to detectors 136-139 by conductormeans 142. In the assembly of FIG. 7, however, detectors 136 through 139merely measure the transmitted light intensity; they do not act asback-scatter or optical time domain locating detectors.

Upon the occurrence of a hydrocarbon fuel leak 143, microbending willoccur at sensor node 122a. Absorbers 131 at sensor node 122a will causemicrobending of optical fibers 123 and 124. The light transmittedthrough optical fibers 126 and 127, however, will not be effected.Detectors 138 and 139 will sense decrease in light intensity transmittedalong fibers 123 and 124, and detectors 136 and 137 will sense acontinuing unchanged light intensity. Computer 141 can immediatelydetermine that the only location along the series of sensor nodes 122which absorber-expanders 131 are coupled to both optical fibers 123 and124 is at sensing node 122a. Thus, the location of leak 143 can bereadily determined by simply sensing a decrease in light transmission atthe combination of fibers 123 and 124, with no decrease in lighttransmission of fibers 125 and 126. This node has the digital code;1100, which is equivalent to the number 3.

A digital output, therefore, can be created simply by couplingabsorber-expanders to a plurality of optical fibers in a manner whichprovides each sensing node 122 with its individual and uniquecombination of sensors. The number of sensing nodes which can be createdis equal to two to the power of the number of fibers less one. The casein which no absorber-expanders are secured to the fibers cannot be used.Missing in FIG. 7 are two digital locations; 1010 and 0101.

One of the advantages of the detection apparatus and method of presentinvention is that the detector can be operated either as an unbiased ora biased detector. Referring to FIG. 10, an unbiased detector will havea Q or quiescent point at zero fiber displacement. When microbendingoccurs, the transmission falls from Q_(u), for example, to the pointP_(u) as a result of microbending or displacement of the fiber.

For biased sensors, Q_(B) is shown in FIG. 10 for a transmission of fluxequal to about 0.7 of the unbiased condition, and this condition can beproduced by microbending of the fiber through a biasing displacement ofx₁. When further microbending occurs as a result of absorber-expanderexpansion, transmission is reduced from Q_(B) to P_(B) while fiberdisplacement increases to x(s).

In serial sensor node arrangements which employ optical time domainlocation, it is necessary to have almost zero insertion loss, that is,to use unbiased sensors with Q_(B) located at maximum transmission andzero displacement. When insertion loss is not a factor of importance,particularly in so-called point sensors, biasing of the sensor can beemployed to increase the signal-to-noise ratio and sensor sensitivity.Such an effect can be used alone or in combination with multiplecoupling rings, as above described.

FIGS. 8 and 9 show one form of sensor biasing which can be used in thedetector assembly method of the present invention. An optical fiber 151has an absorber-expander 152, such as Red Silicone Rubber, coupledthereto by a band 153. Band 153 preferably is formed of a material thatcan be plastically deformed so as to effect biasing of the sensor. Thus,band 153 can have a general trapezoidal shape with one leg 154 extendingaround the expander-absorber 152 and a second leg 156 extending aroundthe absorber-expander in the other direction. Legs 154 and 156 can beseparated from each other to cause microbending of fiber 151. Thus, asleg 156 is moved from the solid line position in FIG. 8 to the phantomline position, optical fiber 151 will experience a microbend to thephantom line position of FIG. 8. Additionally, some deformation occursof absorber-expander 152. Since the containment ring 153 is plasticallydeformable, once the legs are separated, the microbend biasing of thesensor will remain. It will be appreciated that other biasing structuresare suitable for use in the present invention, but the coupling ring 153allows for easy biasing by simply separating legs 154 and 156 until thedesired amount of biasing has been produced. This will cause the biasedquiescent operating point, Q_(B), to be positioned in a manner as shownin FIG. 10.

The hydrocarbon full detector assembly and method of the presentinvention, therefore, employ a hydrophobic, hydrocarbon absorber whichwill expand significantly and is capable of multiple cycles ofabsorption and desorption without breaking down and losing itsstructural integrity. Methyl terminated, silica and iron oxide filled,dimethyl polysiloxane has these characteristics and can be mechanicallycoupled to an optical fiber to cause a decrease in light transmissionupon expansion of the absorber. Microbending or axial fiber misalignmentcan be detected by sensing flux transmission decreases, and OTDRback-scatter or digital sensing node configurations used to determinelocation. Sensitivity can be enhanced by coupling techniques and biasingof the quiescent operating point.

What is claimed is:
 1. A method for detecting the presence of a hydrocarbon analyte in at least one of a liquid and a vapor state comprising the steps of:positioning an optical fiber in a location for detection of said hydrocarbon with an absorber-expander member mechanically coupled to said fiber to produce a change in light transmission in said fiber upon absorption of said hydrocarbon analyte by, and expansion of, said absorber-expander member, said absorber-expander being hydrophobic and being selected to have multiple reversible expansion and contraction cycles upon absorption and evaporation of said hydrocarbon analyte by said absorber-expander; and detecting a change in light transmission in said fiber.
 2. The method as defined in claim 1 and the additional step of:detecting the location along said fiber at which said absorber-expander causes a change in light transmission.
 3. The method as defined in claim 1 and the additional step of:prior to said positioning step, coupling said absorber-expander member to said fiber to produce microbending of said fiber, and during said detecting step, detecting the location of a microbend in said fiber.
 4. The method as defined in claim 1 wherein,said selecting step is accomplished by selecting as an absorber-expander a methyl terminated, silica and iron oxide filled, dimethyl polysiloxane polymer.
 5. The method as defined in claim 1 wherein,prior to said positioning step, said absorber-expander is coupled to a first portion of said optical fiber to displace said first portion relative to a second portion of said optical fiber to produce axial misalignment in said optical fiber.
 6. A method of making a hydrocarbon detection apparatus comprising the steps of:selecting a silicone rubber member as an absorber-expander for a hydrocarbon in at least one of a liquid state and a vapor state, said silicone rubber member being selected to absorb and expand in the presence of said hydrocarbon and to recover and contract to a substantially undegradated condition in the absence of said hydrocarbon; mechanically coupling said silicone rubber member to a fiber optic strand in a manner producing one of microbending and axial misalignment of said strand upon expansion of said silicone rubber member; and optically coupling detection apparatus means to said strand for detection of the occurrence of decreased light transmission along said strand.
 7. The method as defined in claim 6 wherein,said selecting step is accomplished by selecting as an absorber-expander a methyl terminated and iron oxide and silica filled silicone rubber member.
 8. The method as defined in claim 7 wherein,said mechanically coupling step is accomplished by encircling said fiber optic strand with a rigid member and coupling said rigid member to said silicone rubber member for relative displacement of said rigid member and said fiber optic strand against each other to produce microbending.
 9. The method as defined in claim 7 wherein,said mechanically coupling step is accomplished by encircling both said fiber optic strand and said silicone rubber member with a rigid member having a relatively short dimension along said fiber optic strand.
 10. The method as defined in claim 9 wherein,said rigid member is provided by at least one ring.
 11. The method as defined in claim 9 wherein,said rigid member is provided by a wire wound in a spiral around said silicone rubber member.
 12. The method as defined in claim 6 wherein,said selecting step is accomplished by selecting a solid silicone rubber member which absorbs hydrocarbon fuels in both a liquid state and a vapor state.
 13. The method as defined in claim 6 wherein,said selecting step is accomplished by selecting a silicone rubber member which expands upon absorption of hydrocarbons and contracts upon the emission of absorbed hydrocarbons by an amount proportional to the quantity hydrocarbon contacting said silicone rubber member.
 14. The method as defined in claim 6 wherein,said mechanically coupling step is accomplished by encircling said strand and a portion of said rubber member by staple means.
 15. The method as defined in claim 6 wherein,said mechanically coupling step is accomplished by coupling said rubber member to said strand in a manner producing a biasing microbend in said strand prior to any absorption of hydrocarbon by said silicone rubber member.
 16. The method as defined in claim 6 wherein,said optically coupling step is accomplished by optically coupling detection means to said strand suitable for detecting the location of a microbend along said strand.
 17. The method as defined in claim 6 wherein,said mechanically coupling step is accomplished by coupling a plurality of silicone rubber members to a plurality of fiber optic strands at discrete locations producing a digital detection array; and said optical coupling step is accomplished by coupling a detection means to said plurality of strands forming said digital detection array suitable for determining the location of microbending based upon sensing of the presence of microbending on combinations of strands.
 18. The method as defined in claim 6 and the step of:mechanically coupling a temperature control member to a second fiber optic strand, said temperature control member expanding and contracting in proportion to temperature changes by an amount substantially known relative to the expansion and contraction of said silicone rubber member and said temperature control member maintaining its dimensional stability when in contact with said hydrocarbon rubber member for the same temperature changes; optically coupling said second fiber optic strand to said detection means; and positioning said second fiber optic strand so that said temperature control member is closely proximal said silicone rubber member.
 19. The method as defined in claim 6 wherein,said mechanically coupling step is accomplished by mechanically coupling a plurality of absorber-expander members to a single fiber optic strand along a length thereof to provide a series detection array, said absorber-expanders having differing expansion rates upon absorption of a given hydrocarbon.
 20. The method as defined in claim 6 wherein,said mechanically coupling step is accomplished by mechanically coupling a plurality of discrete absorber-expanders to a plurality of fiber optic strands to produce a parallel detection array, said absorber-expanders having differing expansion rates upon absorption of a given hydrocarbon.
 21. The method as defined in claim 6 wherein,said mechanically coupling step is accomplished by mechanically coupling said silicone rubber member to said fiber optic strand to produce microbending of said strand with coupling means multiplying the amount of microbending of said strand upon expansion of said silicone rubber member.
 22. The method as defined in claim 21 wherein,said mechanically coupling step is accomplished by mounting a plurality of side-by-side strand-encircling rigid members on said fiber optic strand.
 23. The method as defined in claim 6 wherein,said fiber optic strand is provided by two strand portions axially aligned for transmission of a signal from one strand portion to the other; and said mechanically coupling step is accomplished by coupling said silicone rubber member to one of said strand portions of said fiber optic strand for displacement of said one of said strand portions transversely relative to the other of said strand portions to produce axial misalignment.
 24. The method as defined in claim 6 wherein,said optic fiber strand is provided by two strand portions axially aligned for transmission of a signal from one strand portion to the other; and said mechanically coupling step is accomplished by coupling said silicone rubber member to both of said strand portions for displacement of said strand portions in opposite directions relative to each other.
 25. A detection apparatus for detecting the presence of at least one of a liquid hydrocarbon and a hydrocarbon vapor comprising:an optical fiber; an absorber-expander member mechanically coupled to said optical fiber to produce a change in transmission of light along said optical fiber upon absorption of said hydrocarbon, said absorber-expander being formed of a hydrophobic hydrocarbon-absorbing rubber material selected to expand upon absorption of said hydrocarbon and selected to retain sufficient structural integrity to permit repetitive use; and detection apparatus optically coupled to said optical fiber for detection of a change in light transmission along said fiber.
 26. The apparatus as defined in claim 25 wherein,said rubber material is selected from the group consisting of silicone rubber, latex rubber and Norprene rubber.
 27. The apparatus as defined in claim 26 wherein,said rubber material is dimethyl polysiloxane having silica and iron oxide fillers therein.
 28. The apparatus as defined in claim 25 wherein,said absorber-expander member is mechanically coupled by a rigid member extending from said absorber-expander member to said optical fiber to produce microbending of said optical fiber.
 29. The apparatus as defined in claim 28, andadhesive means coupling said rigid member to said absorber-expander member.
 30. The apparatus as defined in claim 28 wherein,said rigid member encircles said optical fiber.
 31. The apparatus as defined in claim 30 wherein,said rigid member encircles both said optical fiber and said absorber-expander member.
 32. The apparatus as defined in claim 28 wherein,said rigid member is provided by a ring encircling said optical fiber and a portion of said absorber-expander member.
 33. The apparatus as defined in claim 32 wherein,said optical fiber and said absorber-expander are both cylindrical and oriented with central longitudinal axes substantially parallel and said ring is substantially circular and has a width dimension along said optical fiber which is relatively thin.
 34. The apparatus as defined in claim 33, anda plurality of spaced apart, side-by-side relatively rigid rings each encircling both said optical fiber and said absorber-expander member.
 35. The apparatus as defined in claim 28 wherein,said absorber-expander member is mechanically coupled to said optical fiber by a staple.
 36. The apparatus as defined in claim 28 wherein,said absorber-expander member is mechanically coupled to said optical fiber by a strand wound around both said optical fiber and said absorber-expander member.
 37. The apparatus as defined in claim 36 wherein,said strand is a metallic wire.
 38. The apparatus as defined in claim 25, anda plurality of absorber-expander members each formed as defined for the first-named absorber-expander, said absorber-expanders each being mechanically coupled to said optical fiber at spaced apart positions along the length of said optical fiber in a manner producing microbending of said optical fiber upon absorption of said hydrocarbon.
 39. The apparatus as defined in claim 25 wherein,said absorber-expander member will absorb gasoline, Jet-A fuel and diesel fuel.
 40. The apparatus as defined in claim 39 wherein,said absorber-expander member will absorb said hydrocarbon in both liquid and vapor states.
 41. The apparatus as defined in claim 25 wherein,said optical fiber is provided with a first fiber portion having a first end and a second fiber portion having a second end spaced apart from and optically aligned with said first end for transmission of light from said first fiber portion to said second fiber portion; and said absorber-expander being mechanically coupled to one of said first fiber portion and said second fiber portion to produce displacement of said one laterally relative to the other of the fiber portions by an amount affecting light transmission along said fiber portions.
 42. The apparatus as defined in claim 41 wherein,said absorber-expander is coupled to both said first fiber portion and said second fiber portion and supported for displacement of said first fiber portion and said second fiber portion in opposite directions.
 43. The apparatus as defined in claim 25 wherein,said absorber-expander member is mechanically coupled to said optical fiber by a coupling assembly producing displacement of said optical fiber by an amount greater than the amount of expansion of said absorber-expander member.
 44. The apparatus as defined in claim 25 wherein,said absorber-expander member is mechanically coupled to said optical fiber in a manner biasing said optical fiber with a pre-established microbend at said absorber-expander member.
 45. The apparatus as defined in claim 44, andmounting apparatus mechanically coupling said absorber-expander to said optical fiber, said mounting apparatus producing said microbend.
 46. The apparatus as defined in claim 45 wherein,said mounting apparatus is provided by a ring-like member having a central portion extending around said optical fiber and having a pair of spaced apart leg portions engaging said absorbent-expander member, said leg portions being movable between selected fixed relative positions to pull said central portion toward said optical fiber and establish the amount of microbending biasing said optical fiber.
 47. The apparatus as defined in claim 25 wherein,said absorber-expander member is formed of a rubber material having a known rate of expansion for each of two differing hydrocarbon fuels; and further comprising: an additional optic fiber having an additional absorber-expander member formed of a hydrocarbon-absorbing rubber member mechanically coupled thereto in a manner producing a change in light transmission upon expansion of said additional hydrocarbon-absorbing rubber member, said additional absorber-expander member having a known rate of expansion for each of said two differing hydrocarbon fuels, which rates of expansion differ from the rates of expansion of the first named absorber-expander member; said additional absorber-expander member being positioned proximate said first named absorber-expander member; and said detection apparatus being optically coupled to said additional optical fiber and being formed to quantify the amount of change of light transmission produced upon expansion of the absorber-expanders and including computational apparatus for comparison of the quantity of change of transmission of light from the two optical fibers to determine the hydrocarbon fuel being detected.
 48. The apparatus as defined in claim 25, andan additional optical fiber having an additional rubber member mechanically coupled thereto in a manner substantially identical to coupling of said absorber-expander member to the first-named optical fiber; said additional rubber member having a thermal coefficient of expansion having a known relationship to a thermal coefficient of expansion of said absorber-expander member, and said additional rubber member being substantially incapable of absorbing hydrocarbons absorbed by said absorber-expander member; said additional rubber member being positioned proximate said absorber-expander member; and said detection apparatus being optically coupled to said additional optical fiber and being formed to quantify the amount of change in light transmission produced by expansion of said absorber-expander member and said additional rubber member, and including comparison and computational apparatus for comparing said amount of change and subtracting temperature induced change.
 49. The apparatus as defined in claim 25,said optical fiber has a plurality of substantially similar absorber-expander members mechanically coupled to said optical fiber along a length thereof; a plurality of additional optical fibers positioned proximate the first-named optical fiber; a plurality of additional absorber-expander members substantially similar to the first-named absorber-expander member and mechanically coupled to each of said additional optical fibers; said first-named absorber-expander members and said additional absorber-expander members being coupled along their respective optical fibers in a digital detection array with a different combination of absorber-expanders at each of a plurality of detection sites; said detection apparatus being coupled to each of said additional optical fibers; and computer means coupled to said detection apparatus and formed to be responsive to combinations of signals from said detection apparatus to determine the location of a detected hydrocarbon.
 50. A hydrocarbon detection apparatus comprising:an optical fiber; a methyl terminated, silica and iron oxide filled dimethyl polysiloxane rubber member mechanically coupled to said optical fiber to produce microbending of said fiber upon absorption of at least one of a liquid hydrocarbon and a hydrocarbon vapor by said red silicone rubber; and microbend detection apparatus optically coupled to said fiber to detect the presence of a microbend in said fiber. 